High strength aluminum hot stamping with intermediate quench

ABSTRACT

A method of forming an aluminum alloy part is provided for enhanced formability. The method includes providing a 7xxx series aluminum alloy blank, heating the blank to at least its solvus temperature, and performing an intermediate quench on the blank to a temperature between about 200° C. to about 440° C. at a rate between about 100° C./s to about 500° C./s. The blank is transferred to a stamping die where a secondary quench and forming the blank into a part are carried out, where in one form, the steps of transferring the blank and performing the secondary quench and forming are completed within ten seconds or less. The part may then be optionally artificially aged for increased strength.

FIELD

The present disclosure relates to metal forming, and more specifically to forming parts made from high-strength aluminum alloys such as 7000 series aluminum alloys.

BACKGROUND

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

Commercially successful vehicle body panels have traditionally been made from steel. In recent decades, due to an increasing demand to reduce the weight of vehicle structures while meeting various strength and safety requirements, aluminum body panels have gained interest. The automotive and aerospace industries have predominantly focused on aluminum magnesium (5xxx series) alloys and aluminum-magnesium-silicon (6xxx series) alloys. The 5xxx and 6xxx series aluminum alloys are generally processed by traditional mild steel methods.

Typical 6xxx series aluminum alloys provide a substantial weight savings compared to steel and, with a T6 temper, have a yield stress around or below 350 MPa. However, aluminum-zinc alloys of the 7xxx series can deliver an additional 20-40% potential weight reduction compared to steel. The additional potential weight reduction weight is due to the higher T6 yield strengths, which can exceed 500 MPa. Unfortunately, 7xxx series aluminum alloys does not have a stable T4 temper and generally cannot be stamped at room temperature due to their changing formability.

One economical method of forming 7xxx series aluminum alloys is hot stamping. During the hot stamping process, aluminum sheet is heated, and then simultaneously stamped and quenched in a water-cooled die. This process has been successfully demonstrated and is described in U.S. Pat. No. 8,496,764, which is commonly owned with the present application and incorporated herein by reference in its entirety. However, the formability of 7xxx series aluminum alloys may not be optimal at elevated temperatures.

The present disclosure addresses the issues of forming 7xxx series aluminum alloys at lower temperatures, among other issues related to hot forming 7xxx series aluminum alloys.

SUMMARY

In one form of the present disclosure, a method of forming an aluminum alloy part is provided. The method comprises the steps of providing a 7xxx series aluminum alloy blank, heating the blank to at least its solvus temperature, and performing an intermediate quench of the blank to a temperature between about 200° C. to about 440° C. at a cooling rate between about 100° C./s to about 500° C./s, where the cooling rate is determined from 400° C. to 290° C. Then, the blank is transferred to a stamping die, where a secondary quench and forming the blank into a part are simultaneously performed. Transferring the blank to the stamping die and performing the secondary quench and forming are completed within ten seconds or less.

In variations of this method, the blank is positioned between flat press plates for the intermediate quench, and the secondary quench is performed to a temperature less than 80° C. in a cooled die.

In another variation, the method further comprises a step of artificial aging, wherein the artificial aging may be a PFHT (post forming heat treatment) at a temperature between about 180° C. and about 205° C. for about 30 minutes.

In another variation, the secondary quench is performed to a temperature approximately between 100° C. and 200° C. in a heated die. In a variation of this form, the part is transferred to an elevated temperature enclosure for artificial aging after the secondary quench, wherein the artificial aging may be a PFHT (post forming heat treatment) at a temperature of about 205° C. for about 30 minutes.

The present disclosure also includes parts formed according to the various methods disclosed herein, in addition to vehicles having at least one such part.

In another form of the present disclosure, a method of forming a part is provided that comprises heating a 7xxx series aluminum alloy blank, performing an intermediate quench on the blank to a temperature between about 200° C. to about 440° C., transferring the blank to a stamping die, and performing a secondary quench and forming the blank into a part. In one form, the steps of transferring the blank and performing the secondary quench and forming are completed within ten seconds or less.

In variations of this method, the intermediate quench is performed at a rate between about 100° C./s to about 500° C./s, the secondary quench is performed to a temperature less than 80° C. in a cooled die, the secondary quench is performed to a temperature between about 100° C. and about 200° C. in a heated die, and an additional step of artificial aging after the secondary quench is performed.

In yet another form of the present disclosure, a method of forming an aluminum alloy part is provided that comprises the steps of providing a 7xxx series aluminum alloy blank, heating the blank to at least its solvus temperature, performing an intermediate quench on the blank to a temperature between about 200° C. to about 440° C. at a rate between about 100° C./s to about 500° C./s, transferring the blank to a stamping die, simultaneously performing both a secondary quench on the blank and forming the blank into a part, and artificially aging the part. Transferring the blank to the stamping die and performing the secondary quench and forming are completed within ten seconds or less in one form of the present disclosure.

Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

In order that the disclosure may be well understood, there will now be described various forms thereof, given by way of example, reference being made to the accompanying drawings, in which:

FIG. 1 is a binary phase diagram for an aluminum-zinc alloy system to which the teachings of the present disclosure are applied;

FIG. 2 is an isothermal transformation diagram of various aluminum alloys;

FIG. 3 is a graph of numerous isothermal fracture strain versus lode parameter at a 0.1/s strain rate, according to the teachings of the present disclosure; and

FIG. 4 is a flow diagram for a method for hot stamping 7xxx series aluminum alloys according to the teachings of the present disclosure.

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

While investigating 7xxx series aluminum alloy in-situ material characteristics and properties, the inventors discovered that 7xxx series aluminum alloy formability is not optimal above 440° C. Therefore, to address issues related to hot forming aluminum blanks, the present disclosure provides innovative methods that have been demonstrated to successfully hot form 7xxx series aluminum alloys at or below 440° C. while maintaining high strength, high fracture toughness, and good corrosion resistance.

Referring to FIG. 1, a binary phase diagram for the aluminum-zinc system is shown. The aluminum alpha phase is present across a range of temperatures and compositions. The solid solution region below the Al—Zn eutectic and bounded by the aluminum solvus and zinc solvus is generally more difficult to process than the alpha-phase (a) of aluminum.

Now referring to FIG. 2, an isothermal transformation diagram for various aluminum alloys is shown. An isothermal transformation diagram is only valid for one specific composition of material, and only if the temperature is held constant during the transformation, and strictly with rapid cooling to that temperature. Isothermal transformation diagrams are commonly referred to as Time-Temperature-Transformation (TTT) plots, with lines for various phases referred to as TTT-curves. In this diagram, TTT-curve 42 is for 7075 aluminum, which is being used as illustrative of an entire 7xxx series of aluminum alloys. TTT-curve 42 has a desired process window 44, within which a 7075 aluminum is in an easier to process alpha-phase. (Process window 44 is represented by points A, B, E, and F). If the 7xxx series alloy cools at a rate that avoids intersecting the nose of the TTT curve, then the response of the material to artificial aging is improved and susceptibility to corrosion reduced.

Furthermore, aging an alloy that has been cooled at a rate that avoids intersecting the nose of the TTT curve enables the “peak yield strength” of the alloy. The lower the aging temperature increases the time of the aging process.

In their investigations of hot forming 7xxx series aluminum sheets, the inventors tested, among other relationships, the isothermal relationship between the equivalent plastic strain to failure and a lode parameter within the bounds of process window 44. The isothermal failure strain versus lode parameter curves at a 0.1/s (0.1 s⁻¹) strain rate are plotted in FIG. 3, with T200 representing 200° C., T280 representing 280° C., et al., which are tabulated in Table 1 below:

TABLE 1 Isothermal strain versus load parameter at: FIG. 4 indicia 200° C. T200 280° C. T280 360° C. T360 400° C. T400 440° C. T440 480° C. T480

As shown in FIG. 3, the unexpected results of the present disclosure include an improved formability at elevated temperatures of an equivalent plastic strain to failure above about 1 at a strain rate of 0.1/s both at a lode parameter of about 0 and a lode parameter of about −0.25.

Unexpectedly, T480 strain to failure was lower than T440 but higher than at T200. The T480 strain to failure reduced the bounds of process window 44 to those of process window 44′. (Process window 44′ is bounded by points C, D, H, and G).

Based on these findings, the inventors have discovered that an intermediate quench, followed by a secondary quench that is carried out simultaneously with forming a blank, provides an improvement in forming 7xxx series aluminum at lower temperatures. The intermediate quench quickly cools a blank to an improved forming temperature between about 200° C. and about 440° C., and is further enabled by an intermediate quench rate between about 100° C./s to about 500° C./s, wherein the blank is transferred to a stamping die and the secondary quench is completed within ten (10) seconds or less.

Referring to FIG. 4, this general form of a method for hot stamping a 7xxx series aluminum alloy is illustrated and generally indicated by reference numeral 50. In this method 50, a 7xxx series aluminum alloy blank is provided in step 52. This blank may be from a cold-rolled coil in one form of the present disclosure. In step 54, the blank is heated to at least its solvus temperature, which in this form is about 480° C. In the next step 56, an intermediate quench is performed at a temperature between about 200° C. to about 440° C. In this intermediate quench step 56, the blank may be positioned between flat press plates. Further, the intermediate quench is performed at a rate between about 100° C./s to about 500° C./s in one form of the present disclosure.

In step 58, the warm blank is transferred to a stamping die, which in one form is a cooled die at less than or equal to about 80° C. Alternately, as set forth below, the stamping die is heated to a temperature between about 100° C. and about 200° C. Then, in this step 58, the blank is formed into a part while simultaneously performing a secondary quench on the blank. The secondary quench may thus be performed in a cooled die or a heated die as set forth above. In one form, the steps of transferring the blank to a stamping die and performing the secondary quench and forming are completed within ten seconds or less.

As further shown, the method 50 may also include an optional step of artificial aging of the formed blank in step 60. This optional step of artificial aging is provided after the intermediate and secondary quenches for improved mechanical properties, such as improved tensile strength. When the secondary quench is carried out in a heated die, the time for artificial may be reduced as the secondary quench begins the aging process. Accordingly, the secondary quench temperature ranges from about 100° C. to about 200° C. in the heated die, depending on desired aging characteristics.

In one form, the artificial aging is a PFHT (post form heat treatment) at a temperature between about 180° C. to about 205° C. for 30 minutes. The formed part may be transferred to an elevated temperature enclosure for artificial aging after the secondary quench.

The combination of quenching temperatures, quenching rates, and transfer times provide the desired material properties, such as by way of example, corrosion resistance, fracture toughness, and yield strength. Overall, the present disclosure provides a method in which the formability of 7xxx series aluminum alloys is improved and artificial aging time may be reduced.

Numerous methods of the present disclosure, the form parts and the parts are often incorporated into vehicles.

Unless otherwise expressly indicated herein, all numerical values indicating mechanical/thermal properties, compositional percentages, dimensions and/or tolerances, or other characteristics are to be understood as modified by the word “about” or “approximately” in describing the scope of the present disclosure. This modification is desired for various reasons including industrial practice, manufacturing technology, and testing capability.

The description of the disclosure is merely exemplary in nature and, thus, variations that do not depart from the substance of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure. 

What is claimed is:
 1. A method of forming an aluminum alloy part comprising the steps of: providing a 7xxx series aluminum alloy blank; heating the blank to at least its solvus temperature; performing an intermediate quench of the blank to a temperature between about 200° C. to about 440° C. at a rate between about 100° C./s to about 500° C./s; transferring the blank to a stamping die; and simultaneously performing a secondary quench on the blank and forming the blank into a part, wherein the steps of transferring the blank to the stamping die and performing the secondary quench and forming are completed within ten seconds or less.
 2. The method according to claim 1, wherein the blank is positioned between flat press plates for the intermediate quench.
 3. The method according to claim 1, wherein the secondary quench is performed to a temperature less than 80° C. in a cooled die.
 4. The method according to claim 3 further comprising a step of artificial aging.
 5. The method according to claim 4, wherein the artificial aging is a PFHT (post forming heat treatment) at a temperature between about 180° C. and about 205° C. for about 30 minutes.
 6. The method according to claim 1, wherein the secondary quench is performed to a temperature between about 100° C. and about 200° C. in a heated die.
 7. The method according to claim 6, wherein the part is transferred to an elevated temperature enclosure for artificial aging after the secondary quench.
 8. The method according to claim 7, wherein the artificial aging is a PFHT (post forming heat treatment) at a temperature of about 205° C. for about 30 minutes.
 9. A part formed according to the method of claim
 1. 10. A vehicle having at least one part formed according to claim
 1. 11. A method of forming a part comprising: heating a 7xxx series aluminum alloy blank; performing an intermediate quench of the blank to a temperature between about 200° C. to about 440° C.; transferring the blank to a stamping die; and performing a secondary quench and forming the blank into a part.
 12. The method according to claim 11, wherein the intermediate quench is performed at a rate between about 100° C./s to about 500° C./s.
 13. The method according to claim 11, wherein the secondary quench is performed to a temperature less than about 80° C. in a cooled die.
 14. The method according to claim 11, wherein the secondary quench is performed to a temperature between about 100° C. and about 200° C.
 15. The method according to claim 11 further comprising a step of artificial aging after the secondary quench.
 16. The method according to claim 11, wherein the steps of transferring the blank and performing the secondary quench and forming are completed within ten seconds or less
 17. A method of forming an aluminum alloy part comprising the steps of: providing a 7xxx series aluminum alloy blank; heating the blank to at least its solvus temperature; performing an intermediate quench of the blank to a temperature between about 200° C. to about 440° C. at a rate between about 100° C./s to about 500° C./s; transferring the blank to a stamping die; simultaneously performing a secondary quench on the blank and forming the blank into a part, wherein the steps of transferring the blank to the stamping die and performing the secondary quench and forming are completed within ten seconds or less; and artificially aging the part.
 18. The method according to claim 17, wherein the artificial aging is a PFHT (post forming heat treatment) at a temperature between about 180° C. and about 205° C. for about 30 minutes.
 19. The method according to claim 17, wherein the secondary quench is performed to a temperature less than about 80° C. in a cooled die.
 20. The method according to claim 17, wherein the secondary quench is performed to a temperature between about 100° C. and about 200° C. in a heated die. 